[1] The headword properly only means the second of these things, but see next note.
[2] Herodotus wrote in Ionic Greek, a dialect in which ορf- becomes ουρ- . Additionally, Ionic Greek lost rough breathings via a process called de-aspiration or psilosis. Thus the word ὅρος (from *ὅρfος )--meaning "border"--appears as οὖρος in Ionic and therefore looks identical to the word that means "guardian". See LSJ s.v. ὅρος (web address 1) for the forms in Ionic and other dialects, with an example from Herodotus. The present entry thus reveals something interesting about ancient understanding of the morphology of Greek dialects. For a summary of Herodotus' Ionic dialect, see Flower and Marincola 2002, 44-48.
[3] This non-classical Greek word, a neuter plural, appears in the Glossae in Herodotum omicron2 s.v. οὔροι and is glossed in Hesychius omicron1278 as "the things dividing the land" (τὰ χωρίζοντα τὴν γῆν ).